The purpose of this project is to identify, quantitate and characterize an SV40 induced 53,000MW (p53) cellular protein in embryonal carcinoma cells (EC), embryonic cells and in various other transformed cells. Undifferentiated EC cells and primary cells prepared from midgestation mouse, rat and hamster embryos all expressed the protein P53 without SV40 infection. The 2D tryptic peptide map obtained from EC and embryo cells showed that this protein is similar to SV40 induced p53 and is conserved evolutionarily in both embryonic and in SV40 transformed cells. The amount of the protein determined quantitatively in both cells were found to be half that of SV40 transformed cells. The amount of p53 decreased with differentiation of EC cells, and with the increasing age of the embryo. The protein was also detected in various tumorigenic mouse cells such as L cells, neuroblastoma cells, placental cells and 3T12 cells. In the established mouse cells prepared from AL/N strain mouse embryo, the expression of p53 did not correlate with tumorigenicity. The turnover of this protein was found to be rapid in tumorigenic and in non-tumorigenic cells whereas in SV40 transformed cells the protein was found to be stable due to its interaction and complex formation with large T antigen.